<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <link rel="stylesheet" href="style.css">
</head>
<body>
  <div class="wrapper">
    <div class="box px">I am 200px wide</div>
    <!--  10vw is 10 percent of the width of the viewport
    vw:viewport width
    -->
    <div class="box vw">I am 10vw wide</div>
    <!-- em relative to font size  -->
    <div class="box em">I am 10em wide</div>
  </div>
</body>
</html>